1. Field of the Invention
The present invention relates to identifying magnetic heads used in gang or group head environments. More particularly, the invention concerns a method, apparatus, and article of manufacture which allows a head designated from a group of heads, used in a direct access storage device having a storage medium, to be identified during read and write procedures even though the storage medium being read from or written to contains no head number information.
2. Description of the Related Art
"Format Efficiency" refers to the amount of each track in a disk drive devoted to storing user data. The format efficiency may be improved by reducing the overhead. There are a number of contributors to overhead in the format of fixed block architecture disk drives. Some of these, such as synchronization fields, are required for reading the data. Others, such as error correcting codes (ECC) and sector servo, offset their overhead by allowing the areal density to be increased. One contributor to the overhead that does not increase the areal density is the header of an ID field. "ID" is a collective term used to refer to cylinder-number, head-number and record-number fields and information typically stored in the ID field. An alternative to using ID fields is the No-ID sector format which eliminates the ID fields and all the information they contain from the track format, providing a marked improvement in the format efficiency.
Sector Format
Typically, each track on a disk of a DASD is divided into a number of data sectors and servo sectors. The servo fields contain the positioning information used to locate the head over a given track. The user data is stored in the data fields, each with an associated ID field. The ID fields contain information which identifies the data sector and other information, such as flags to indicate defective sections or head number identification.
The majority of disk drives manufactured today use an addressing scheme where the data sectors are identified to the host system by a logical block number (LBN). In operation, the host computer sends a list of logical block numbers to be written or read. The disk drive converts these values into zone, cylinder, head and sector (ZCHS) values. The servo system seeks to the desired zone, cylinder and head, and the disk drive begins reading ID fields until a match is found. Once the appropriate ID field has been read, the drive may then read or write the following data field. The use of ID fields allows great flexibility in the format, and provides a simple mechanism for handling defects. However, there are substantial costs associated with the use of ID fields. The ID fields themselves can occupy up to 10% of a track--space that would otherwise be used to store data. Further, since the disk drive must read the ID field for each sector prior to a read or write operation, additional space is required to allow for write-to-read recovery prior to each ID field. Such write-to-read fields can occupy over 5% of a track.
Furthermore, the use of PRML (partial response, maximum likelihood) digital data detection channels may result in increasing the size of ID fields. PRML channels allow disk drives to operate at higher linear densities. But, PRML digital channels may require replacing the cyclical redundancy checking (CRC) bytes typically used to detect ID field errors with a greater number of ECC bytes capable of correcting errors. To solve these ID field problems, No-ID sector formatting was developed.
No-ID sector format
No-ID sector format uses the servo control system to locate physical sectors, and a defect map stored in RAM to identify logical sectors. This allows the disk data controller to perform a new operation--converting logical block numbers (LBN) to physical block numbers (PBN). The LBN is just a number from 0 to the number of addressable blocks on the disk drive. The PBN is a number from 0 to the number of physical blocks on the disk drive, but with defective and space sectors mapped out. Once the PBN is computed, it may be converted to the exact Zone/Cylinder/Head/Sector (ZCHS) value for the sector. Since the defect information is known in advance, the proper logical block is guaranteed to be located at the computed ZCHS. The defect map is stored in a compressed format and optimized for small size rapid lookup. The servo system is used to locate the physical sector, based upon knowledge of the track formats in each zone. This information includes the locations of any data field splits due to embedded servo, which are also stored in RAM. This method effectively eliminates the need for an ID field on a track.
Gang Servo Writing
Before disk drives used the No-ID architecture, gang servo-writing--writing more than one head at a time--posted no data integrity problems. The ID field containing the head number was written when a file was formatted on a storage disk and later used during normal file operation to check for proper head selection.
In No-ID architecture, the head number information for a non-ganged head is stored on a storage disk when the servo field is written by the head. Later, verification of the head's number can be had by reading back the stored information.
However, in the case of gang servo writing, where multiple heads write to the storage disk at the same time, each head will have the same head number stored on the disk. Currently there is no economical way to write different head numbers to each of these heads. Accordingly, the system controlling the head writing or reading is unable to verify which head was selected because the data contained in the servo field for various heads is identical. This creates a large degree of data integrity exposure.
For the foregoing reasons, there is a great need for an inexpensive method and apparatus that allows the number of a ganged head to be verified without resorting to reading data from a storage disk to verify the head selection.